Health care personnel, swine and poultry farm workers, airline and public transportation workers, and others may be infected by a variety of viruses that can be transmitted through air. Infection control experts divide transmission of infectious agents through air into two categories, e.g., "droplet transmission" and "airborne transmission". Conventional wisdom suggests that most transmission of infectious viruses occurs by droplet transmission. However, more recent research indicates that at least some viruses can be transmitted by the airborne route. Remarkably, no one has assessed the sizes of particles with which viruses are associated in occupational environments. Our research will answer this critical question: `Is airborne viral transmission a more important mechanism than conventional wisdom suggests?" During this study, methods will be developed to quantify the amount of virus associated with airborne particles of different sizes and the viability of these viruses in any environment. The first step is to separate airborne particles into different size ranges. Impactors and differential mobility analyzers will be evaluated for their ability to achieve this separation. The second step is to analyze the particles divided into each size range for the amount of virus present within them. The quantification of live viruses (and a bacteriophage) will be accomplished by inoculation of appropriate cell systems (or bacterial host for bacteriophage). Molecular methods such as polymerase chain reaction (PCR) will also be used as a semiquantitative method to detect both live and inactivated viruses. After methods are validated in the laboratory, they will be used in swine barns and health care facilities to measure particle sizes with which viable and non-viable viruses are associated and to which workers may be exposed. This improved understanding of virus behavior will influence the procedures and technology used to prevent virus transmission in health care facilities, animal facilities, public venues, and other workplaces. Recent research shows that some viruses can be transmitted by the airborne route. We will develop methods to quantify the amount of viruses associated with airborne particles of different sizes and the survivability of these viruses in the environment. These methods will then be used in swine barns and health care facilities to measure particle sizes with which viable and non-viable viruses are associated and to which workers may be exposed. This improved understanding of virus behavior will help prevent virus transmission in health care facilities, animal facilities, public venues, and other workplaces.